1. Field of the Invention
The present invention relates to a plasma processing apparatus, a focus ring, and a susceptor.
2. Description of the Related Art
Widely known examples of plasma processing apparatuses include CVD apparatuses, etching apparatuses, ashing apparatuses and so on. Such a plasma processing apparatus has a plasma processing chamber in which is installed a susceptor on which a wafer W, i.e. an object to be processed, is mounted. As shown in FIG. 16, the susceptor is comprised of a disk-shaped electrostatic chuck 51 on which the wafer W is mounted, and a focus ring 52 that is made of only a conductive material or only a dielectric material and is disposed on an outer peripheral edge of an upper surface of the electrostatic chuck 51.
When carrying out plasma processing on a wafer W, the wafer W is mounted on the electrostatic chuck 51, and then while maintaining the processing chamber at a predetermined degree of vacuum, in a state with the processing chamber filled with a processing gas, for example a processing gas comprised of C4F8, O2 and Ar, the wafer W is fixed onto the electrostatic chuck 51 using electrostatic attraction, and high-frequency electrical power is applied to the electrostatic chuck 51, thus producing a plasma from the processing gas in the processing chamber. The plasma is focused over the wafer W by the focus ring 52 on the electrostatic chuck 51, and hence predetermined plasma processing (e.g. dry etching (reactive ion etching: RIE) processing) is carried out on the wafer W. At this time, the temperature of the wafer W increases due to being subjected to the dry etching processing, but the wafer W is cooled by a cooling mechanism built into the electrostatic chuck 51. In this cooling, a backside gas such as helium gas having excellent heat transfer ability is made to flow from the upper surface of the electrostatic chuck 51 toward the rear surface of the wafer W, thus improving the heat transfer ability between the electrostatic chuck 51 and the wafer W, whereby the wafer W is cooled efficiently.
On the other hand, a gap of which a width is several microns exists between the upper surface of the outer peripheral edge of the electrostatic chuck 51 and a rear surface of the focus ring 52 due to undulations on the rear surface of the focus ring 52 caused by surface roughness thereof. When the processing chamber is put into a vacuum state by reducing the pressure therein, this gap becomes in a vacuum state, and thus forms a vacuum thermal insulation layer; the heat transfer ability between the electrostatic chuck 51 and the focus ring 52 thus becomes low, and hence the focus ring 52 cannot be cooled efficiently as with the wafer W, and as a result the temperature of the focus ring 52 rises more than the temperature of the wafer W. Due to this increase in the temperature of the focus ring 52, an outer peripheral portion of the wafer W becomes hotter than an inside portion thereof, and hence at the outer peripheral portion the etching characteristics become poor, i.e. the hole penetration property (the perpendicular degree of a hole formed by etching relative to a surface of the wafer W) deteriorates, the etching selectivity drops and so on.
Moreover, in recent years, there have been rapid advances in increasing the diameter of wafers W and in ultrafine processing, and hence it has become that a large number of devices are produced from a single wafer W. There are thus cases in which devices are also produced from the outer peripheral portion of a wafer W. It is thus necessary to prevent the temperature of the focus ring 52 from increasing, thus preventing the etching characteristics at the outer peripheral portion of the wafer W from deteriorating.
To prevent the temperature of the focus ring 52 from increasing, it is necessary to improve the heat transfer ability between the focus ring and the electrostatic chuck; as a susceptor in which this heat transfer ability is improved, as shown in FIG. 17, there is known a susceptor 66 comprised of an electrostatic chuck 62 having coolant channels 61 built therein, a focus ring 63 disposed on an outer peripheral edge of a wafer W mounting surface of the electrostatic chuck 62, a heat transfer medium 64 interposed between the electrostatic chuck 62 and the focus ring 63, and a fixing jig 65 that presses and thus fixes the focus ring 63 against the electrostatic chuck 62 (see Japanese Laid-open Patent Publication (Kokai) No. 2002-16126 (FIG. 1)).
According to the susceptor 66, the heat transfer medium 64 is deformed by a load applied thereto from the fixing jig 65 via the focus ring 63, and thus fills the gap between the electrostatic chuck 62 and the focus ring 63, and hence the degree of close contact between the electrostatic chuck 62 and the focus ring 63 is increased, whereby the heat transfer ability between the electrostatic chuck 62 and the focus ring 63 is improved.
Moreover, as an etching apparatus in which the temperature of the focus ring is prevented from increasing, as shown in FIG. 18, there is known an etching apparatus 75 comprised of an electrostatic chuck 72 provided inside a reaction chamber 71, a focus ring 73 provided at a periphery of an upper portion of the electrostatic chuck 72, and cooling means (a cooling unit) 74 provided along a lower surface of the focus ring 73, wherein the cooling unit 74 has a substrate 74a that is made of a material having good thermal conductivity and is provided in close contact with the lower surface of the focus ring 73, and a coolant pipe 74b that is provided inside the substrate 74a and through which a coolant is circulated (see Japanese Laid-open Patent Publication (Kokai) No. H11-330047 (FIG. 1)).
Moreover, as another etching apparatus, there is known an apparatus in which a backside gas such as helium (He) gas having excellent heat transfer ability is made to flow from an upper surface of an electrostatic chuck toward a rear surface of a focus ring, thus diffusing the backside gas through a vacuum gap existing between the electrostatic chuck and the focus ring and hence filling this vacuum gap with the backside gas, whereby the heat transfer ability between the electrostatic chuck and the focus ring is improved.
Furthermore, to improve the heat transfer ability between a focus ring and an electrostatic chuck, the degree of close contact between the focus ring and the electrostatic chuck may be improved. To this purpose, there is known an etching apparatus having an electrode built into the electrostatic chuck in a manner facing the focus ring. According to this apparatus, a voltage is applied to the electrode, whereby the electrode attracts the focus ring to the electrostatic chuck by electrostatic attraction, thus improving the degree of close contact between the focus ring and the electrostatic chuck.
However, with the susceptor 66 described above, in addition to the component parts of a conventional susceptor, the heat transfer medium 64 and the fixing jig 65 are necessary, and hence the initial cost increases. Furthermore, the fixing jig 65 is exposed to the plasma, and hence is consumed as plasma processing is carried out repeatedly, resulting in regular maintenance becoming necessary. There is thus also a problem of the maintenance cost increasing.
Moreover, the coolant channels 61 built into the electrostatic chuck 62 collect not only heat from the focus ring 63 but also heat from the fixing jig 65, and hence there is a problem that the efficiency of cooling of the focus ring 63 cannot be improved as much as hoped.
Moreover, with the etching apparatus 75 described above, again the cooling unit 74 is necessary, and hence the initial cost increases; furthermore, if the cooling unit 74 is exposed to the plasma, then the cooling unit 74 will be consumed as plasma processing is carried out repeatedly, resulting in regular maintenance becoming necessary, and hence there is also a problem of the maintenance cost increasing.
Furthermore, with the other etching apparatus, the vacuum gap that exists between the electrostatic chuck and the focus ring has low thickness, and hence it is not possible to make the backside gas diffuse through the vacuum gap sufficiently, and as a result the heat transfer ability between the electrostatic chuck and the focus ring cannot be improved sufficiently. There is thus a problem that the efficiency of cooling of the focus ring cannot be improved as much as hoped.
Moreover, in general, plasma processing is comprised of a plurality of steps, and the magnitude of the high-frequency electrical power used to generate the plasma may be changed from step to step, and hence the temperature of the focus ring may change. However, the pressure of the backside gas and the voltage applied to the electrode built into the electrostatic chuck are not changed from step to step but rather are constant throughout the plasma processing, and hence the heat transfer ability between the focus ring and the electrostatic chuck does not change. There is thus a problem that changes in the temperature of the focus ring caused by changes in the magnitude of the high-frequency electrical power cannot be suppressed, and hence the efficiency of cooling of the focus ring cannot be improved.